1. Field of the Invention
The present invention relates to digital telephone systems. More particularly, the present invention relates to a novel and improved method and apparatus for using state determination from an echo canceller to control various functional blocks in a digital telephone system.
2. Description of the Related Art
Transmission of voice by digital techniques has become widespread, particularly in cellular telephone and PCS applications. This, in turn, has created an interest in improving speech processing techniques. Three of such techniques include the addition of echo cancellers, noise suppressors, and voice encoders/decoders, or vocoders, to existing elements of digital telephone systems.
Echo cancellers are used to diminish undesired echo signals caused by impedance mismatches in land-based telephone networks, or in the case of mobile telephones, echo caused by acoustic coupling between speaker and microphone in "hands free" telephones. Vocoders are used to remove natural redundancies of speech in a digitized signal in order to reduce data transmission rates and consequently the amount of information being transmitted over a given transmission channel. Noise suppressors are used to minimize background noise. Echo cancellers, vocoders, and noise suppressors are presently used together in digital telephone systems both in land-based applications and in mobile systems.
There are two types of echo cancellers, the network echo canceller and the acoustic echo canceller. An example of a typical network echo canceller is disclosed in U.S. Pat. No. 5,307,405 entitled "NETWORK ECHO CANCELLER", which is assigned to the assignee of the present invention and incorporated by reference herein. A network echo canceller cancels the echo produced in a telephone network. A land-based telephone is connected to a central office by a two wire line to support transmission in both directions. For calls farther than about 35 miles, the two directions of transmission must be segregated onto physically separate wires, resulting in a four-line wire. The device that interfaces the two-wire and four-wire segments is known as a hybrid. An impedance mismatch at the hybrid results in an echo which must be removed by a network echo canceller. Acoustic echo cancellers are used in teleconferencing and hands-free telephony applications. An acoustic echo canceller eliminates acoustic echo resulting from the feedback between a loudspeaker and a microphone.
In a typical digital telephone system, speech is converted from an analog signal to digital PCM samples by an A/D converter. In a typical embodiment, a data rate of 64 kbps is chosen in order to retain good voice quality. Once the speech signal has been digitized, it can be manipulated to achieve certain benefits, such as maximization of system capacity, speech quality enhancement, noise suppression, and minimization of transmission errors.
After the speech signal has been converted to PCM samples, undesired echo can be removed by an echo canceller, background noise can be minimized by a noise suppressor, and data compression can be performed by a vocoder before modulation and upconversion for transmission. An example of a variable rate vocoder is disclosed in U.S. Pat. No. 5,414,796 entitled "VARIABLE RATE VOCODER", which is assigned to the assignee of the present invention and incorporated by reference herein. The encoded speech signal can be modulated by any number of techniques, including TDMA, CDMA, or analog modulation. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled "SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS," which is assigned to the assignee of the present invention and incorporated by reference herein. Combining the echo canceller with the vocoder and noise suppressor has certain benefits as well as problems associated with it.
One problem with introducing an echo canceller into the front end electronics of a digital telephone system is that it alters the speech signal to the other functional blocks due to its location in the system relative to the other functional blocks. By placing the echo canceller first in the chain of functional blocks, the noise suppressor and vocoder must make background noise calculations based on an echo-canceled signal rather than actual background noise. If the echo canceller does not remove all of the echo from the speech signal, the residual echo can cause errors in the background noise calculations performed by the noise suppressor and vocoder.
Herein, a mobile user is referred to as the near-end speaker and the land-based user is referred to as the far-end speaker. A typical vocoder may contain a noise suppressor whose function is to remove background noise from the near-end speech signal. An example of a typical noise suppressor is disclosed in U.S. Pat. No. 4,811,404 entitled "NOISE SUPPRESSION SYSTEM", which is assigned to Motorola, Inc. and incorporated by reference herein. Noise suppression is performed by calculating an estimate of the actual background noise energy during periods when the near-end speaker is silent. A problem occurs if the near-end speaker is silent and the far-end speaker is talking. In the mobile telephone, the far-end speaker's voice can be acoustically coupled from the speaker to the microphone, resulting in an echo that will be heard by the far-end speaker unless it is removed. In a land-based system, near-end speech can be coupled onto the far-end speaker's voice signal due to the impedance mismatch in the hybrid discussed above. An echo canceller is used to eliminate the echo, but because of limitations of the echo canceller, the echo will not be completely removed. A noise suppressor placed after the echo canceller may interpret the residual echo as background noise and update the background noise estimate based upon the residual echo. This corrupts the background noise estimate, resulting in degraded noise suppression. The vocoder will suffer by providing a poor estimate of the background noise to a synthesized noise generator in the system. In addition, the vocoder's encoding rate decisions will be adversely affected.
It is therefore an object of the present invention to prevent erroneous background noise updates in the noise suppressor and the vocoder encoder when the near-end speaker is silent and the far-end speaker is active.
It is another object of the present invention to use the state determination signal from the echo canceller to control other functional elements within a digital telephone system, such as a tone detector, a transmission mute function, and an adaptive equalizer.